What powdery mildew looks like
Powdery mildew is a fungal disease that looks like a white dust on the surface of the leaf. You can wipe the dust off with a finger. Within a few days it comes back.
Other things can also leave white marks on a cannabis leaf. Your first job is to work out which one you have. The section “Telling it apart from other white marks” below shows you how.
You manage this disease across the whole room. Treating one plant is not enough. If one plant has it, the room conditions suit the fungus, and every plant in that room is at risk.
Why it appears in a room that looks dry
Most fungal leaf diseases need a film of water on the leaf before they can start. Powdery mildew does not. That is why it appears in rooms that are otherwise dry. It is also why growers who have already fixed their watering are surprised to find it.
Three conditions usually come together:
- High humidity at night.
- Poor air movement inside a dense canopy (the layer of leaves and branches at the top of the plants).
- A large gap between day and night temperature.
The third condition feeds the first. When air cools at night, its relative humidity rises, even if no water has been added. A large drop in temperature after lights-off can therefore push humidity up around the leaves.
What to do:
- Measure where the leaves are. Hang a thermo-hygrometer with min/max memory at canopy height, between the plants and not on the wall. (A thermo-hygrometer measures temperature and humidity. Min/max memory means it records the highest and lowest reading.) Read it every morning for 2 weeks. This shows you the real night humidity and temperature in the canopy, which a wall meter often misses.
- Keep night humidity under control. Keep night-time humidity as low and as steady as your equipment allows. Run the dehumidifier and fans through the whole lights-off period, not only during the day. Humidity is usually highest at night, so that is when the fungus has its best chance.
- Narrow the day–night temperature gap. Keep the gap as small as your setup allows. In an indoor room the lamps stop producing heat at lights-off. If the air conditioner keeps the same setting, the room cools sharply. Raise the air conditioner set point or lower its output at lights-off, then check the min/max reading the next morning to see if the gap got smaller.
- Open up the canopy. Remove small lower shoots and leaves that get almost no light (see defoliation). Leave space between plants. Aim a fan so that the leaves inside the canopy move slightly. Moving air carries away the layer of humid air around each leaf.
Translator’s note: In Thailand and much of Southeast Asia, the rainy season keeps outdoor air very humid. Each time you open the door or bring in fresh air, the room’s humidity can jump. Outdoor and greenhouse crops in the north and northeast often get hot days and cool nights in the cool season, which gives exactly the large day–night gap described above. Power cuts at night stop fans and dehumidifiers when the risk is highest. After every outage, check the min/max reading to see how high humidity went. If outages are common where you grow, consider a UPS (uninterruptible power supply, a battery backup that keeps equipment running during a cut) or a generator for the fans and dehumidifier. For help sizing equipment, see Dehumidifier and Air-Conditioner Sizing for a Thai Flowering Room.
Telling it apart from other white marks
Powdery mildew grows as mycelium (the fine threads that make up the body of the fungus). The mycelium sits on the leaf surface, not inside the leaf. That makes it easy to see and easy to test with a wipe.
The same white look can come from three other things:
- Residue from a foliar spray (any product sprayed onto the leaves).
- Spots from hard water, which is water with a lot of dissolved minerals. When the droplets dry, the minerals stay on the leaf.
- Trichomes, the tiny resin glands on the flowers and the small leaves around them.
Check in this order:
- Wipe test. Rub the white patch lightly with a clean fingertip or a cotton bud. Powdery mildew comes off as a loose powder.
- Recheck test. Mark the wiped leaf with a piece of tape on its stem. Check it again after a few days. If the white comes back in the same place, it is most likely powdery mildew. Spray residue and water spots do not come back unless you spray or wet the leaf again.
- Compare with your spray record. Did the marks appear right after spraying, and only where droplets landed? Then residue or hard-water spotting is more likely than the fungus.
- Use a magnifier. Look with a jeweller’s loupe or a phone macro lens. Trichomes stand up from the surface like tiny stalks and feel sticky. They do not wipe off as powder.
These checks keep you from spraying a plant that has no disease. They also help you catch real cases early. Wash your hands or change gloves after touching an infected leaf, so you do not carry the fungus to other plants.
Why spraying close to harvest is not a solution
Anything applied to a flower stays on it through drying. The flower is then inhaled, and it may be sent to a lab for testing. A late spray is therefore a legal and testing problem as well as a growing problem.
What to do:
- Plan before flowering starts. Once flowers have formed, rely mainly on controlling the room climate. Set up your humidity, airflow and temperature measures before you switch to flowering.
- Use only registered products, exactly as the label says. Use only products registered with the Department of Agriculture (กรมวิชาการเกษตร). Follow the label on dose, crop and timing. That includes the pre-harvest interval (the minimum number of days between the last application and harvest) when the label gives one. For what this means under Thai standards, see What You May Legally Spray Under Thai GACP.
- If mildew appears late in flowering, carefully remove the affected leaves into a closed bag so you do not shake spores into the air. Then fix the room conditions described above.
- Record every application. Write down the product, the dose and the date. If a lab report or an inspection raises questions, this record is your answer. For what labs test for and what cannot be removed afterwards, see Pesticide Residues on Cannabis.
Translator’s note: In Thailand, cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568. If your flower goes to a lab, residue from a late spray can show up in the results. For how to read those results, see How to Read a Cannabis Certificate of Analysis.
Where to start
Related elsewhere on the forum: Integrated Pest Management for Cannabis, in the Diseases, Insects and Mites section.
Posting your case
If you think you have powdery mildew, start a topic in this category. Include a close-up photo, your wipe test and recheck results, your night humidity and temperature readings, the growth stage, and anything you sprayed in the last few weeks. With these details other growers can tell whether it is the fungus and what in your room is causing it.